1. Field of the Invention
The invention relates to a semiconductor device, and more particularly to a pad structure of an image sensor device.
2. Description of the Related Art
A complementary metal-oxide-silicon (CMOS) image sensor is a device for converting optical images into electrical signals. The CMOS image sensor uses MOS transistors as switching devices for sequential transfer of electrical signals. A CMOS image sensor has certain advantages over a charge coupled device (CCD) image sensor. CMOS image sensors, for example, have lower manufacturing costs less consume less power than CCD image sensors. CCD image sensors are also more difficult to fabricate than CMOS image sensors. Moreover, random access is not possible with CCD image sensors, but is possible with CMOS image sensors. CMOS fabrication techniques have been under continuous development since the 1990's and signal-processing algorithms have also been continuously developed, yielding improved CMOS image sensors.
FIG. 1 is a cross sectional view of a conventional CMOS image sensor disclosed in U.S. Pat. No. 6,964,916 B2. The CMOS image sensor has a post-passivation pad portion 224 on the top of a bond pad 222, forming a bond pad structure (including bond pad 222 and portion 224) having a height greater than the height of the bond pad, and having a step height less than that of the bond pad. The post-passivation pad portion 224 is slightly higher than a top of a planarization layer 204a. The conventional CMOS image sensor has a planar topography, but the stack height of the conventional CMOS image sensor is still too high. FIG. 1b is a cross sectional view of another conventional CMOS image sensor disclosed in U.S. Pat. No. 6,369,417 B1. The pad open portion 310 formation is carried out after forming the color filter 305, the planarized photoresist 306 and the micro-lens 307, it is possible to prevent the surface of the metal line 301 form being damaged or contaminated. The pad open portion 310 of the conventional CMOS image sensor is electrically connected to the top metal 301, and the stack height of the conventional CMOS image sensor must still be reduced.
If the stack height of the CMOS image sensor is too high, sensitivity of the sensor may be affected, resulting in more material absorbing light intensity under a light pass. Also, higher stack height results in problematic cross talk due to increased light diffraction.
Thus, a CMOS image sensor with lower stack height is desirable.